1. Field of the Invention
The present invention relates to a solid electrolyte fuel cell and more particularly to its cell structure, i.e., electrode/solid electrolyte/separator assembly and a supporting structure therefor.
2. Description of the Related Art
Solid electrolyte fuel cells use zirconia as the electrolyte and operate at high temperatures near 1,000.degree. C. and, hence, high efficient, high power generation is expected to occur and no reforming of raw fuel nor maintenance of the electrolyte is required. The exhaust gas from the fuel cells is at a high temperature so that exhaust heat can be used for various purposes.
Recently, research and development of solid electrolyte fuel cells have increased. From their structure, solid electrolyte fuel cells are divided into two groups, i.e., cylindrical and planar ones. Ceramics are used as a major material in the both types of the fuel cells.
FIG. 1A is an enlarged view which shows a main part of a conventional cylindrical solid electrolyte fuel cell, and FIG. 1B is a perspective view which shows a module including the main part of the conventional cylindrical solid electrolyte fuel cell. In FIGS. 1A and 1B, a gas-tight housing 10, which is provided with a fuel gas supply manifold 12, an oxidant gas supply manifold 14, and an exhaust gas conduit 16, contains a plurality of annular or tubular fuel cells 18. One of the tubular fuel cells 18 is illustrated in FIG. 1A. The fuel cell 18 has a closed end 20 facing an end wall 22 of the housing 10, and an open end 24 opposite to the closed end 20. An air conduit 26 having an opening (not shown) at its lower end is inserted in the cavity of the tubular fuel cell 18 through the open end 24 thereof. The air conduit 26 is supported on a tube sheet 28 and the fuel cell 18 is supported on a porous barrier 30. Each fuel cell 18 has one or more interconnectors 32 through which the plurality of fuel cells are electrically interconnected to each other. A fuel inlet or generating chamber 34 extends between the end wall 22 of the housing 10 and the porous barrier 30. A preheating chamber 36 extends between the porous barrier 30 and the tube sheet 28. An air inlet chamber 38 extends between the tube sheet 28 and an upper wall 40 of the housing 10 and the tube sheet 28. The tube sheet 28 and the porous barrier 30 need not be gas-sealed. Fuel gas and oxidant gas, e.g., air, are supplied into the fuel gas supply manifold 12 and the oxidant gas supply manifold 14, respectively. The air supplied from the air supply manifold 14 flows into the air inlet chamber 38 and then into the air conduit 20, flows out of the opening of the conduit 20, flows back in the cavity of the tubular fuel cell 18, and flows out of the open end 24 of the fuel cell into the preheating chamber 36. On the other hand, the fuel gas supplied from the fuel gas supply manifold 12 is introduced into the generating chamber 34 through fuel inlet ports 42 provided in the end wall 22 of the housing 10 and unreacted fuel flows through the porous barrier and a gap between the porous barrier and the fuel cell. Excess air and unreacted fuel gas are exhausted from the exhaust gas conduit 16.
The apparatus shown in FIGS. 1A and 1B substantially corresponds to the solid electrolyte fuel cell described in Japanese Patent Application Laying-open No. 113561/1982 corresponding to U.S. Pat. No. 4,395,468 to Isenberg, assigned to Westinghouse Electric Corporation (cf., especially FIGS. 1 and 3). The fuel cell is considered to be a model which would be closest to a practically acceptable one. The major feature of the above-mentioned conventional fuel cell is that in principle, it allows a free thermal expansion and needs no gas seal.
FIG. 1C is a perspective view showing another conventional annular solid electrolyte fuel cell. In FIG. 1C, an annular fuel cell 50 includes a porous support tube 52 which is comprised of calcia stabilized zirconia forming a porous wall. A thin film porous air electrode, or cathode 54 surrounds the outer periphery of the porous support tube 52. The cathode 54 is comprised of LaMnO.sub.3. A layer of gas-tight solid electrolyte 56, which is comprised of 8 mole % yttria-stabilized zirconia, surrounds the cathode 54. A substantial part of the solid electrolyte 56 is surrounded by a porous anode 58 comprised of Ni-zirconia cermet. An interconnector 60, which is comprised of LaCrO3, is provided on a selected part of the solid electrolyte 56. Further, a cell interconnection part 62, which is comprised of an Ni felt, is provided on the interconnector 60 to connect the fuel cell with a similar adjacent fuel cell. In the fuel cell arrangement shown in FIG. 1C, during operation, air flows through the center of the annular cell 50 and fuel gas such as hydrogen gas (H2) passes over the exterior. Oxygen from air diffuses through the porous support 52 and cathode 54, and fuel gas diffuses through the anode 58. The reactant gases react through the solid electrolyte 56, generating products such as water vapor, carbon dioxide, heat and electrical energy. Current flows from the outer anode 58 to the cathode 54 and then to the interconnection part 62 as indicated by arrows 64. That is, current flows in a circumferential direction or along the surface of the thin electrode 54. As a result, the above-described fuel cell has a defect that resistance loss in the electrode 54 is more than negligible so that the output density of the fuel cell is decreased. The above-described fuel cell substantially corresponds to the fuel cell solid electrolyte fuel cell configuration described in U.S. Pat. 4,490,444 (Isenberg, assigned to Westinghouse Electric Corporation) corresponding to Japanese Patent Application Laying-open No. 130381/1982.
FIG. 2 is an exploded perspective view which shows a conventional planar solid electrolyte fuel cell. In FIG. 2, reference numeral 70 designates an assembly of a single cell 72 comprised of a plate of solid electrolyte 74 and electrodes 76 and 78, and a separator plate 80, the single cell 72 and the separator plate 80 being alternately superimposed one on another. The separator plate 80 is provided with a plurality of parallel grooves 82 on one main surface thereof and another plurality of parallel grooves 84 on the opposite surface thereof. The grooves 82 and 84 are arranged at right angles. Manifolds 86 are attached to respective side surfaces of the assembly 70. The manifolds are provided with conduits 88, respectively, through which reactant gases are introduced. The different reactant gases flow through the grooves 82 and 84, respectively. In the planar fuel cell arrangement shown in FIG. 2, current flows through the cell in a direction perpendicular to the surface of the cell as indicated by an arrow 89 so that it is in principle possible to reduce the flow path length of current as compared with the annular fuel cell described above. As a result, it can be expected that the planar fuel cell has a resistance loss smaller than that of the tubular fuel cell and, hence, an output density higher than that of the tubular fuel cell. However, since the planar solid electrolyte fuel cell is generally constructed such that the fuel cell and the separator plate are alternately superposed one on another in a direction of gravitation and in addition manifolds are attached on the sides of the resulting assembly, the fuel cells and the separators tend to suffer from damages due to stress by their weight as well as thermal stress. Furthermore, it is very difficult to achieve a gas seal between the cells and the separator plates, between the manifolds and the cells, and between the manifolds and the separator plates. The above-described conventional planar solid electrolyte fuel cell substantially corresponds to the solid electrolyte fuel cell disclosed in Japanese Patent Application Laying-open No. 75262/1992 (cf. especially FIG. 5 of the publication). In FIG. 2 of the present application, manifolds are additionally illustrated for better understanding.
FIG. 3 is a cross sectional view which shows another conventional planar solid electrolyte fuel cell as disclosed in U.S. Pat. No. 4,874,678 (Reichner, assigned to Westinghouse Electric Corporation) corresponding to Japanese Patent Application Laying-open No. 169878/1989. As shown in FIG. 3, a solid oxide electrolyte electrochemical cell stack configuration comprises a plurality of flattened, elongated, connected cell combinations 90. Each cell combination contains an interior electrode or air electrode 91. The air electrode 91, which serves as a substrate, is comprised of a flattened, elongated porous material having a top surface and a plurality of interior gas feed conduits or chambers 92, through its axial length, with electrolyte 93 contacting the interior electrode 91, and exterior electrode 94 contacting the electrolyte. A major portion of the air electrode top surface 95 is covered by an interconnection material 96. Further, each cell has at least one axially elongated, electronically conductive, flexible, porous, metal fiber felt material 97 in electronic connection with the air electrode 91 through contact with a major portion of the interconnection material 96. The interior gas feed chamber 92 is closed at one end, and a gas feed tube 98 is inserted in the chamber 92. Oxygen gas (one of the reactant gases) fed through the gas feed tube 98 passes through the space between the outer surface of the feed pipe and the surface of the interior gas feed chamber 92 along the cell length to exhaust at the open end of the chamber. Fuel gas flows along the outside surface of the electrochemical cell combination. The electrochemical cell combinations are electrically connected through the metal fiber felt strip 97. Thermal stresses generated in the cell combinations due to changes in temperature are alleviated by the above-described arrangement which allows free expansion and contraction. In the air electrode 91, current flows in a direction perpendicular to the top surface thereof. However, the conventional electrochemical cell combination configuration described above has disadvantages, such as the current flow path in the electrochemical cell combination cannot be reduced since the gas feed pipes are located within the gas feed chambers so that the inner resistance of the cell increases thus lowering the output density of the cell.